gpu: drm: replace of_graph_get_next_endpoint()

From DT point of view, in general, drivers should be asking for a
specific port number because their function is fixed in the binding.

of_graph_get_next_endpoint() doesn't match to this concept.

Simply replace

	- of_graph_get_next_endpoint(xxx, NULL);
	+ of_graph_get_endpoint_by_regs(xxx, 0, -1);

Link: https://lore.kernel.org/r/20240202174941.GA310089-robh@kernel.org


Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/87frob3neo.wl-kuninori.morimoto.gx@renesas.com